📚 Electric Current for CCEA IGCSE Physics | 电流考点精讲
Electric current is one of the most fundamental concepts in IGCSE Physics. To succeed in the CCEA exam, you must clearly understand what current is, how to measure it, how it behaves in different circuits, and how to use the equation linking current, charge and time. This article breaks down every essential point with clear explanations and exam-style hints, supported by bilingual notes to reinforce your learning.
电流是 IGCSE 物理中最基础的概念之一。想在 CCEA 考试中取得成功，你必须清楚电流是什么、怎样测量它、它在不同电路中如何表现，以及怎样使用电流、电荷和时间的关系式。本文将每一个关键点都拆解清楚，配合考试风格的提示，并以双语讲解强化你的理解。
1. What is Electric Current? | 什么是电流？
Electric current is the rate of flow of electric charge. In a circuit, charges move from one point to another, and the current tells us how quickly this movement occurs.
电流是电荷流动的速率。在电路中，电荷从一点移动到另一点，电流告诉我们这种移动有多快。
In a solid metal conductor, the moving charges are negatively charged electrons. These electrons are loosely held and can drift through the metal lattice when a voltage is applied.
在固态金属导体中，移动的电荷是带负电的电子。这些电子受到的束缚较弱，当施加电压时，它们能在金属晶格中漂移。
The symbol for current is I. You will often see it in circuit equations and on ammeters.
电流的符号是 I。你会在电路方程和电流表上经常见到它。
2. Conventional Current vs Electron Flow | 传统电流方向与电子流动
Conventional current is defined as flowing from the positive terminal to the negative terminal of a cell or battery. This historical convention was established before the electron was discovered and is still used in circuit diagrams and analysis.
传统电流方向被定义为从电池的正极流向负极。这个历史惯例是在发现电子之前确立的，至今仍用于电路图和分析中。
In reality, electrons move in the opposite direction – from the negative terminal, through the circuit, and back to the positive terminal. This is called electron flow.
实际上，电子沿相反方向移动 —— 从负极出发，经过电路，回到正极。这称为电子流动。
When solving circuit problems or reading diagrams, always use conventional current. Diodes and other semiconductor symbols are drawn with arrows pointing in the direction of conventional current.
在解电路问题或阅读电路图时，请始终使用传统电流方向。二极管和其他半导体符号上的箭头指向传统电流的方向。
3. Conditions Needed for an Electric Current | 产生电流的必要条件
For a current to exist, there must be a closed conducting loop – a complete circuit. If there is a break anywhere in the loop, charges cannot flow around the circuit, so the current stops.
要让电流存在，必须有一个闭合的导通回路 —— 即一个完整的电路。如果回路中任何地方有中断，电荷就无法在电路中流动，电流便会中止。
A source of potential difference (a cell, battery or power supply) is also required to provide the push that moves charges. Without a p.d., charges have no reason to drift.
还需要一个提供电动势的电源（电池、蓄电池或电源）来提供推动电荷的力。没有电势差，电荷就没有理由漂移。
Conductors must contain free charge carriers. Metals have a sea of free electrons; electrolytes contain mobile ions. Insulators like plastic or glass have very few free carriers, so they block current flow.
导体必须含有自由电荷载流子。金属中有大量自由电子；电解质中含有可移动的离子。塑料或玻璃等绝缘体自由载流子极少，因此它们阻碍电流通过。
4. Unit of Current: The Ampere | 电流的单位：安培
The SI unit of electric current is the ampere, symbol A. One ampere is defined as one coulomb of charge passing a point in one second.
电流的国际单位制 (SI) 单位是安培，符号为 A。1 安培定义为每秒钟有 1 库仑的电荷通过某一点。
In circuits, we often deal with smaller currents, so milliamperes (mA) and microamperes (μA) are common: 1 A = 1000 mA, and 1 mA = 1000 μA.
在电路中，我们常遇到更小的电流，因此毫安 (mA) 和微安 (μA) 很常用：1 A = 1000 mA，1 mA = 1000 μA。
Being comfortable converting between these units is essential for calculation questions. Always check that you are using amperes when using I = Q / t.
熟练进行这些单位之间的换算，对计算题至关重要。使用 I = Q / t 时，务必确保电流的单位是安培。
5. Measuring Current: The Ammeter | 测量电流：安培表
An ammeter is the instrument used to measure the current flowing through a component. The circuit symbol for an ammeter is a circle with a letter A inside it.
安培表是用于测量流过某一元件电流的仪器。安培表的电路符号是一个圆圈，里面有一个字母 A。
To measure the current correctly, the ammeter must be connected in series with the component. This ensures the same current that flows through the component also passes through the meter.
要正确测量电流，安培表必须与被测元件串联。这能确保流过该元件的电流也经过电表。
A common dangerous mistake is to connect an ammeter directly across a power supply (in parallel). Because an ammeter has a very low resistance, this creates a short circuit, causing a very large current that can blow a fuse or damage the meter.
一个常见的危险错误是将安培表直接并联在电源两端。由于安培表内阻极低，这样会形成短路，产生极大的电流，可能烧断保险丝或损坏电表。
6. Current in Series Circuits | 串联电路中的电流
In a series circuit, there is only one path for charges to take. Therefore, the current is exactly the same at every point in the loop.
串联电路中，电荷只有一条通路。因此，回路中每一点的电流都完全相同。
If you insert several ammeters at different positions around a series circuit, they will all give the same reading: I₁ = I₂ = I₃.
如果你在串联电路的不同位置接入几个安培表，它们的读数全都相同：I₁ = I₂ = I₃。
The current does not get ‘used up’ as it goes through lamps or resistors. Only energy is transferred; the number of coulombs per second stays constant throughout the circuit.
电流在经过灯泡或电阻器时并不会被“用光”。只有能量发生了转移；每秒通过的库仑数在整个电路中保持恒定。
7. Current in Parallel Circuits | 并联电路中的电流
In a parallel circuit, the current splits at junctions to flow through different branches. The total current drawn from the supply is equal to the sum of the currents in all parallel branches: Itotal = I₁ + I₂ + … .
在并联电路中，电流在节点处分成几条支路流动。从电源流出的总电流等于所有并联支路电流之和：Itotal = I₁ + I₂ + … 。
Charge is conserved, so the total number of coulombs entering a junction each second must equal the total number leaving the junction each second. This is a direct consequence of the law of conservation of charge.
电荷守恒，所以每秒进入节点的库仑总数必须等于每秒离开节点的总数。这是电荷守恒定律的直接结果。
The current in each branch depends on the resistance of that branch – a branch with smaller resistance carries a larger current. You are not required to calculate individual branch currents from resistance alone at this stage, but you should be able to reason qualitatively.
每条支路中的电流取决于该支路的电阻 —— 电阻较小的支路通过的电流较大。现阶段不要求仅凭电阻计算各支路电流，但你应能进行定性分析。
8. The Equation Linking Current, Charge and Time | 电流、电荷和时间的关系式
The definition of current as the rate of flow of charge is expressed mathematically by the equation:
电流作为电荷流动速率的定义，可用数学公式表示为：
I = Q ÷ t
where I is current in amperes (A), Q is electric charge in coulombs (C), and t is time in seconds (s). You can also write it as I = Q / t.
其中 I 是电流，单位为安培 (A)；Q 是电荷，单位为库仑 (C)；t 是时间，单位为秒 (s)。你也可以写成 I = Q / t。
This equation can be rearranged to find charge: Q = I × t, or time: t = Q ÷ I. Make sure you memorise all three forms or be confident rearranging the equation.
该公式可变形成求电荷：Q = I × t，或求时间：t = Q ÷ I。务必记住这三种形式，或熟练掌握公式变形。
9. Worked Examples Using I = Q ÷ t | 运用 I = Q ÷ t 的例题
Example 1: A steady current of 0.4 A flows through a filament lamp. Calculate the charge that passes through the lamp in 5 minutes.
例题 1： 一个 0.4 A 的恒定电流流过一盏灯丝灯泡。计算 5 分钟内通过灯泡的电荷量。
First, convert the time to seconds: t = 5 × 60 = 300 s. Then using Q = I t gives Q = 0.4 × 300 = 120 C.
首先将时间转换为秒：t = 5 × 60 = 300 s。然后使用 Q = I t 得到 Q = 0.4 × 300 = 120 C。
Example 2: How long must a current of 2.5 A flow to transfer 500 C of charge?
例题 2： 要使 500 C 的电荷被转移，2.5 A 的电流需要流动多长时间？
Rearrange the equation: t = Q ÷ I = 500 ÷ 2.5 = 200 s. Always leave your answer in seconds unless the question asks for minutes.
变形公式：t = Q ÷ I = 500 ÷
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